Organic electroluminescence device

ABSTRACT

A luminescence device is constituted by a substrate, a first electrode disposed on the substrate, at least one organic luminescence function layer disposed on the first electrode, a second electrode disposed on the above at least one organic luminescence function layer, and an oxygen absorbent disposed between the substrate and the second electrode or between the first and second electrodes. To the luminescence device, a voltage is applied between the first and second electrodes to cause phosphorescence from at last one layer constituting the above-mentioned at least one organic luminescence function layer preferably containing the oxygen absorbent. The oxygen absorbent may be formed in a layer disposed at a region other than pixel portions.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an organic electroluminescence (EL)device used as a light-emitting device for flat panel displays,projection displays, printers, etc.

Self-emission type devices for the flat panel display, such as a plasmaemission device, a field emission device, and an electroluminescence(EL) device have attracted notice in recent years.

Of these self-emission type devices, the EL device is classified into anorganic EL device and an inorganic EL device.

The inorganic EL device is a self-emission device utilizing luminescencebased on collisional excitation. On the other hand he organic EL deviceis a self-emissin device of a carrier injection-type utilizingluminescence caused at the time of recombination of electron and holecarried into a luminescence layer.

With respect to the organic EL device, T. W. Tang et al. havesubstantiated in 1987 that it is possible to realize a high-luminanceluminance (1000 cd/m²) at a lower voltage (10 volts) by utilizing alamination structure comprising a film of fluorescent metal chelatecomplex and a film of diamine-based molecules.

The self-emission device of carrier injection-type has extensivelystudied and developed. Specifically, organic EL devices usinglow-molecular weight materials for green luminescence or area color-typeluminescence of, e.g., red (R), green (G) and blue (B) have beencommercialized. Further, at present, a full-color organic EL device hasbeen extensively developed.

FIGS. 1 and 2 are respectively a schematic sectional view of anembodiment of an ordinary organic EL device of lamination organicluminescence function layer-type. Referring to FIGS. 1 and 2, theorganic EL device comprises a cathode 11 or 21, an anode 14 or 25, and alamination organic luminescence function layer, including a luminescencelayer 12 or 23 and a hole transport layer 13 or 24, disposed between thecathode 11 or 21 and the anode 14 or 25. In FIG. 2, an electrontransport layer 22 is disposed between the cathode 21 and theluminescence layer 23.

Examples of a material for the cathode 11 or 21 may generally includemetals having smaller work functions, such as aluminum, aluminum-lithiumalloy and magnesium-silver alloy. Further, as a material for the anode14 or 25 as a transparent electrode, it is possible to use anelectroconductive material having a lager work function, such as ITO(indium tin oxide), thus allowing light emission via the transparentelectrode.

The organic luminescence function layers disposed between the cathode 11or 21 and the anode 14 or 25 may have a two-layer structure includingthe luminescence layer 12 and the hole transport layer 13 as shown inFIG. 1 and a three-layer structure including the electron transportlayer 22, the luminescence layer 23, and the hole transport layer 24 asshown in FIG. 2.

The hole transport layer 13 or 24 has a function of efficientlyinjecting holes from the anode 14 or 25 to the luminescence layer 12 or23. The electron transport layer 22 has a function of efficientlyinjecting electrons from the cathode 21 to the luminescence layer 23.Further, the hole transport layer 13 or 24 and the electron transportlayer 22 also have functions of confining electrons and holes in theluminescence layer 12 or 23, respectively (i.e., carrier blockingfunctions), thus enhancing a luminescence efficiency.

A commercially available liquid crystal display device as a full-colorflat panel-type display device effects full-color image display byusing, e.g., color filters.

On the other hand, the organic EL device allows self-emission of primarycolors of red (R), green (G) and blue (B) by appropriately selectingmaterials constituting luminescence layers, thus advantageously providea resultant EL device with a high responsiveness and a wide viewingangle.

In order to realize a sufficiently practical full-color display device,it is necessary to provide a luminescence device excellent in luminance,chromaticity, and luminescence efficiency for respective colors (R, G,B).

Generally, it is difficult to satisfy the above luminescencecharacteristics in combination in the case of forming luminescencelayers for R, G, B of a single material. In order to obviate thedifficulty, an organic EL device of a colorant doping-type wherein ahost material is doped with a fluorescent organic compound (fluorescentcolorant) to shift its emission center wavelength is generally employed.More specifically, referring again to FIGS. 1 and 2, at least onematerial for constituting organic luminescence function layers (the holetransport layer, the electron transport layer, the luminescence layer,etc.) is used as a host and is doped with a small amount of thefluorescent colorant to utilize luminescence from the fluorescentcolorant. In this case, it is possible to use a colorant exhibiting ahigher fluorescence efficiency, thus allowing improvement in quantumefficiency and a wide latitude in selection of respective luminescencecolors.

With respect to such a fluorescent colorant-doped organic EL device,Murayama et al. has proposed a luminescence device using an aluminumquinolinol complex doped with a quinacridone derivative, whereby amaximum luminance of at least 100,000 cd/m² has been achieved (Preprintfor 54th Meeting of the Applied Physics of Japan, 1127 (1993).

In the organic EL device, as described above, holes and electronscarried into a luminescence layer are recombined to form an excitationstate, thus causing luminescence.

Accordingly, in the organic EL device, excitation energy is required tosuppress consumption thereof in steps other than a luminescence step inorder to efficiently utilize the excitation energy as that forluminescence in a step of transition of organic material moleculescontributing to luminescence from an excitation state to a ground state.

There-are several factors for such energy consumption, whereby devicecharacteristics are adversely affected considerably. For example, aluminescence efficiency is lowered to result in a dark luminescencestate or luminescence per se is not caused to occur.

Generally, the organic EL device is considerably affected by moisture(or water content). Specifically, the organic EL device is accompaniedwith a defective region causing no luminescence therein (called “darkspots”) due to degradation or deterioration of a metal electrode and/oradsorption of water content to impurities in some cases. Such dark spotsare gradually enlarged with time by the influence of water content, thusadversely affecting the life of the organic EL device.

Further, in addition to the influence of water content, it has beengenerally known that oxygen entering the organic EL device oxidizeselectrodes and/or organic materials used therein, thus loweringdurability of the organic EL device.

In order to overcome the problem, Japanese Laid-Open Patent Application(JP-A) 7-169567 has disclosed such a device structure that a sealingstructure including an oxygen absorbent layer for oxygen absorption andan oxygen barrier layer with little oxygen permeability is formedoutside an organic EL device structure.

In the organic EL device of this type, however, a fluorescence organiccompound is used as a luminescence center material as in theabove-described conventional EL devices, thus merely providing a lowerquantum efficiency and a lower luminance relative to power supply.

This may be attributable to the following mechanism.

Carriers, such as electrons and holes, injected from a pair ofoppositely disposed electrodes are recombined within a luminescencelayer formed of a organic luminescence function material to placemolecules of the organic luminescence function material in an excitedstate (higher energy state) (herein, such molecules are referred to as“excitons”). The excited state includes an excited single state and anexcited triplet state determined based on a difference in spin state. Inthe case of an ordinary fluorescent organic compound, only fluorescencefrom the excited singlet state is observed at room temperature and nophosphorescence from the excited triplet state is observed.

In this case, according to the statistical method, excitons placed inthe single state and those placed in the triplet state may presumably beformed in a ratio of 1:3. For this reason, a theoretical limit of aninternal quantum efficiency in the case of an organic EL device using afluorescent material has been considered to be 25%. Further, in the caseof an organic EL device of a simple lamination-type, an efficiency fortaking emitted light out is ca. 20%, thus resulting in an externalquantum efficiency of ca. 5% as an upper limit value. Indeed, theconventional organic EL devices at best provide an external quantumefficiency of ca. 5%.

In order to improve the external quantum efficiency of the organic ELdevice, Baldo et al. has proposed an organic EL device exhibiting anexternal quantum efficiency increased up to ca. 8% by using a metalcomplex containing iridium as a center metal and a phenylpyrimidineligand (“Applied Physics Letters”, Vol. 75, No. 1, pp. 4- (1999)). Thehigher external quantum efficiency may be attributable to a particulartriplet state of the iridium complex exhibiting a strongerphosphorescence. Based on the stronger phosphorescence, it is possibleto efficiently utilize excitons in the triplet state occupying theremaining 75% of all the excitons. As a result, the internal quantumefficiency can be estimated to be increased up to 100% as thetheoretical limit.

As described above, in recent years, an organic EL device using aphosphorescent material has attracted notice as a high-efficiencyself-emission device.

The organic EL device utilizing phosphorescence is, however, accompaniedwith a serious problem of oxygen quenching (quenching due to oxygen)causing deterioration in initial performance or that with time of theresultant EL device. According to our study, this problem isparticularly noticeable in th case of the organic EL device using aphosphorescent material compared with that using a fluorescent material.

This may be attributable to the following factors (1) and (2).

(1) A ground state of oxygen is a triplet state, thus readily causingenergy transfer or transition between the oxygen triplet state and anexcited triplet state of molecules of a luminescent material to take theexcitation energy of the luminescence material molecules (i.e., oxygenquenching).

(2) The life of an excited triplet state is longer than that of anexcited single state by at least three digits. For this reason, a timefrom the energy excitation step to a subsequent luminescence step islonger in the case of utilizing phosphorescence, thus resulting in anincreased probability of consumption of the excitation energy due toenergy transition with no luminescence including the oxygen quenching.

As a result of our study, it has been confirmed that the presence ofoxygen in an organic EL device particularly using a phosphorescentmaterial adversely affects not only an initial luminescence luminancebut also the life of the resultant EL device, such as lowerings inluminescence luminance and luminescence efficiency when the EL device iscontinuously or discontinuously driven for a certain period of time. Ithas been also found that such lowerings in luminescence luminance andefficiency are considerably pronounced when compared with theconventional organic EL device using a fluorescent material.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide an organicelectroluminescence device having solved the above-mentioned problems.

A specific object of the present invention is to provide ahigh-efficiency organic electroluminescence device using aphosphorescent material capable of suppressing the influence of oxygento improve an initial luminance and present a deterioration inperformances with time in combination.

According to the present invention, there is also provided aluminescence device, comprising: a substrate, a first electrode disposedon the substrate, at least one organic luminescence function layerdisposed on the first electrode, a second electrode disposed on said atleast one organic luminescence function layer, and an oxygen absorbentdisposed between the substrate and the second electrode.

According to the present invention, there is also provided aluminescence device, comprising: a substrate, a first electrode disposedon the substrate, at least one organic luminescence function layerdisposed on the first electrode, a second electrode disposed on said atleast one organic luminescence function layer, and an oxygen absorbentdisposed between the first electrode and the second electrode.

In the luminescence device (organic EL device) of the present invention,the above-mentioned oxygen absorbent may preferably be contained in atleast one layer constituting the organic luminescence function layer byblending or co-vapor deposition or disposed in proximity to the organicluminescence function layer using a phosphorescent material, thusallowing absorption and/or adsorption of oxygen within the luminescencedevice to effectively suppress not only a lowering in initialluminescence luminance but also a deterioration in performances (e.g.,.luminescence efficiency) of the device with time at repetitive use.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are respectively a schematic sectional view of an ordinaryorganic electroluminescence device.

FIG. 3 is a schematic sectional view of an embodiment of theluminescence device organic electroluminescence device according to thepresent invention.

FIG. 4 is a schematic sectional view of an embodiment of theluminescence device of a simple matrix-type according to of the presentinvention.

FIG. 5 is a time chart of a drive waveform for driving the luminescencedevice employed in Example 2 appearing hereinafter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The organic electroluminescence device according to the presentinvention basically has a structure shown in FIG. 3.

Referring to FIG. 3, an organic EL device includes: a substrate 1, ananode 2, at least one organic luminescence function layer 2 including anorganic luminescence function layer 4 containing an oxygen absorbent, acathode 5, a sealing housing (or casing) 6, an adhesive resin 7, and ahygroscopic agent 8. The substrate 1, the sealing housing and theadhesive resin together constitute a sealing means.

The substrate 1 may preferably be formed of a transparent heat-resistantmaterial, such as glass.

On the substrate 1, the anode 2 as a transparent electrode is formed.Examples of a material for the anode (transparent electrode) 2 mayinclude those exhibiting a higher work function, such as CuI, ITO(indium tin oxide) and SnO₂, so as to improve a hole injectionefficiency from the anode.

On the anode 2, at least one organic luminescence function layer 3 atleast containing a luminescent material. The organic luminescencefunction layer 3 may have a single-layer structure or a lamination-layerstructure which includes two layers comprising a luminescence layer andan electron transport layer or a hole transport layer; three layerscomprising a luminescence layer, an electron transport layer and a holetransport layer; and four or more layers including the above layers. Theorganic luminescence function layer 3 may be formed by vacuum depositionor spin coating.

The luminescence layer contained in the organic luminescence functionlayer 3 comprises a phosphorescent material, such as a metal complexcontaining a heavy metal (as a center metal) having a large spin-orbitinteraction (e.g., Ru, Rh, Pd, Os, Ir Pt, Au, etc.). Representativeexamples of the phosphorescent material may include iridium complexeshaving a ligand, such as phenylpyridine o or thienyl-pyridine; andplatinum porphyrin derivatives.

The oxygen absorbent used in the present invention may be contained in apart of the organic luminescence function layer 3 or the entire organicluminescence function layer 3. In FIG. 3, the oxygen absorbent iscontained in the organic luminescence function layer 4 constituting thethree-layer type organic luminescence function layer 3 as a part of theorganic luminescence function layer 3.

Examples of a material for the oxygen absorbent may include metalshaving a lower work function such as alkali metal and alkali earthmetal; and compounds including metal oxides, such as iron oxide.

Herein, the oxygen absorbent refers to a substance capable ofselectively absorbing and/or adsorbing oxygen physically or chemically.

The organic luminescence function layer containing the oxygen absorbentmay be formed by co-vacuum deposition of the oxygen absorbent with theorganic luminescence function material (such as a luminescent material)or by spin-coating a solution of an oxygen absorbent powder in anappropriate solvent (such as chloroform).

On the organic luminescence function layer 3, the cathode 5 as a metalelectrode is formed, thus preparing an organic EL device having aprincipal structure.

Examples of a material for the cathode 5 may preferably include thosehaving a lower work function, such as Mg-Ag ally, Al, and Al-Li alloy,so as to improve an electron injection efficiency from the cathode.

In order to hermetically seal up the above-prepared organic EL device soas to block ambient air, the housing 6 is bonded to the substrate 1 at aperiphery thereof so as to enclose the organic EL device by using theadhesion resin 7.

Examples of a material for the housing 6 may preferably include amoisture barrier material, such as glass or metal. Examples of amaterial for the adhesive resin 7 may preferably include epoxy resin andUV (ultraviolet)-curable resin.

At the inner surface of the sealing housing 6, the hygroscopic agent 8may preferably be disposed in order to suppress the influence ofmoisture (water content). Examples of a material for the hygroscopicagent 8 may preferably include oxides, such as calcium oxide and bariumoxide.

With a spacing between the sealing housing 6 and the organic EL deviceof the present invention, inert gas such as rare gas (e.g., argon gas)or nitrogen gas may preferably be filled in order to remove gasesadversely affecting the organic EL device including oxygen.

In the present invention, the oxygen absorbent may be disposed not onlywithin the organic EL device but also within the sealing housing at thesame time.

FIG. 4 shows another embodiment of the luminescence device (organic ELdevice) according to the present invention.

Referring to FIG. 4, in this embodiment, the organic EL device includesan oxygen absorbent 53 formed on a substrate 51 in a stripe shape at aspacing between stripe-shaped first electrodes 52. On the firstelectrode 52, an organic luminescence function layer 54 containing aluminescence layer is disposed. On the organic luminescence functionlayer 54, stripe-shaped second electrodes 55 ar disposed so as tointersect the first electrodes 52 to form a matrix of pixels each at anintersection.

Hereinbelow, the present invention will be described more specificallybased on Examples.

EXAMPLE 1

On a 1.1 mm-thick glass substrate (20×25 mm), a ca. 100 nm-thicktransparent electrode (anode) of ITO (indium tin oxide) was formed bysputtering, followed by patterning.

On the ITO electrode, four organic luminescence function layers (firstto fourth layers) were successively formed in the following manner.

First, on the ITO electrode, a 40 nm-thick first layer (hole transportlayer) of α-NPD (N4,N4′-di-naphthalene-1-yl-N4,N4′-diphenylbiphenyl-4,4′-diamine) shown below was formed byvacuum deposition (2.7×10⁻³ Pa).

On the first layer, a 40 nm-thick second layer (luminescence layer) of aluminescent material comprising CPB (4,4′-N,N′-dicarbazole biphenyl)shown below and Ir(ppy)₃ (fac tris(2-phenylpyridine)iridium) (CBP:Ir(ppy)₃=93:7 by weight) by co-vacuum deposition (2.7×10⁻³ Pa) at acontrolled deposition rate.

On the second layer, a 10 nm-thick third layer (exciton diffusionprevention layer) of BCP (2,9-dimethyl-4,7-diphenyl-1-,10-phenanthroline(Bathocuproin)) shown below doped with 1 wt. % of Mg (magnesium) byco-vacuum deposition (2.7×10⁻³ Pa) at a controlled deposition rate.

On the third layer, a 20 nm-thick fourth layer (electron injectionlayer) of Alq3 (tris(8-hydroxyquinoline)aluminum (aluminum-quinolinolcomplex)) shown below doped with 1 wt. % of Mg by co-vacuum deposition(2.7×10⁻³ Pa) at a controlled deposition rate.

In this example, as an oxygen absorbent, Mg was used in the third andfourth layers in a form of co-deposited film.

On the thus-formed four organic luminescence function layers, a 150nm-thick Al electrode (cathode) was formed by vacuum deposition(2.7×10⁻³ Pa) with a hard mask of stainless steel so as to provide amatrix of pixels each having an area of 3 mm² at each intersection withthe patterned ITO electrode, thus preparing an organic EL deviceaccording to the present invention.

The thus-prepared organic EL device was placed in a glove box filledwith nitrogen gas, and a sealing housing of glass was bonded thereto byusing an epoxy resin adhesive. At that time, CaO powder (hygroscopicagent) was sealed in a spacing between the EL device and the sealinghousing.

Characteristics of the EL deice were measured at room temperature byusing a microammeter (“Model 4140B”, mfd. by Hewlett-Packard Co.) for acurrent-voltage characteristic and a luminance meter (“Model BM 7”, mfd.by Topcon K. K.) for a (luminescence) luminance. As a result, the ELdevice of the present invention showed a good rectificationcharacteristic.

More specifically, when the organic EL device was driven by applying avoltage of 12 volts between the ITO electrode (anode) and the Alelectrode (cathode), the EL device showed a current density of 9 mA/cm²and a luminance of 1900 cd/m². At that time, a higher external quantumefficiency of 5.7% was obtained.

Then, when a change in luminance from an initial luminance of 100 cd/m²of the EL device was measured by continuously driving the EL device at aconstant current, the EL device exhibited a luminance half-life (a timefor decreasing the initial luminance ((100 cd/m²) to ½ thereof (50cd/m²)) of 498 hours.

COMPARATIVE EXAMPLE 1

An organic EL device was prepared and evaluated in the same manner as inExample 1 except that PCB (for the third layer) and Alq 3 (for thefourth layer) were not doped with Mg (i.e., the oxygen absorbent was notused at all).

The resultant organic EL device exhibited a current density of 8.4mA/cm² (under application of a voltage of 12 volts), a luminance of 1200cd/m², an external quantum efficiency of 3.9%, and a luminance half-lieof 272 hours, thus providing EL characteristics inferior to those of theEL device prepared in Example 1.

EXAMPLE 2

A simple matrix-type organic EL device as shown in FIG. 4 was preparedin the following manner.

On a 1.1 mm-thick glass substrate 51 (75×75 mm), a ca. 100 nm-thicktransparent electrode 52 of ITO (anode) was formed by sputtering,followed by patterning in a stripe form including 100 lines each havinga width of 100 μm and a spacing (with an adjacent line) of 40 μm.

On the stripe ITO electrode 52, an oxygen absorbent of Mg was formed byvacuum deposition with a mask in a stripe pattern 53 at respectivecenter portions of the spacing of the stripe ITO electrode 52 so as tohave a width of 10 μm and a thickness of 50 nm (for each stripe Mglayer).

On the ITO electrode 52 and the stripe pattern 53 of Mg, four organicluminescence function layers 54 were formed in the same manner as inExample 1 except that Mg (as the oxygen absorbent) was not used at all.

Then, on the organic luminescence function layers 54, a lamination metalelectrode (cathode) 55 comprising a 10 nm-thick Al-Li alloy layer (Li:1.3 wt. %) and a 150 nm-thick Al layer (disposed on the Al-Li alloylayer) was formed by vacuum deposition (2.7×10³ Pa), followed bypatterning in a stripe form including 100 lines (each having a width of100 μm and a spacing of 40 μm) arranged so as to intersect the stripeITO electrode 52 at right angles, thus preparing an organic EL deviceincluding a matrix of pixels (100×100 pixels) each at an intersection ofthe lines of ITO and metal electrodes.

The thus-prepared EL device was placed in a glove box filled withnitrogen gas, and a sealing housing of glass (having an area sufficientto enclose the entire EL device) was bonded to the EL device by using anepoxy resin adhesive. At that time, CaO powder (hygroscopic agent) wassealed in a spacing between the EL device and the sealing housing.

The EL device (100×100 pixels) was then driven in a simple matrix manner(frame frequency: 30 Hz, interlace scanning manner) in the glove box byapplying a drive waveform of 7-13 volts (scanning signal voltage: 10volts, data signal voltage: ±3 volts) as shown in FIG. 5.

As a result, it was confirmed that the EL device provided smooth motionpicture images.

When the EL device (including 100×100 lines) was driven in aline-sequential manner, the EL device showed an initial luminance of 34cd/m² in a whole area-luminance state. Further, when the EL device wascontinuously driven, a resultant luminance half-life was 460 hours.

COMPARATIVE EXAMPLE 2

A simple matrix-type organic EL device was prepared and evaluated in thesame manner as in Example 2 except that the stripe Mg layer 53 was notformed (i.e., the oxygen absorbent was not used at all).

The thus-prepared EL device exhibited an initial luminance of 19 cd/m²and a luminance half-life of 202 hours, thus being considerably inferiorin EL characteristics to those of the EL device prepared in Example 2.

As described hereinabove, according to the present invention, it ispossible to provide a high-efficiency organic EL device (luminescencedevice) expected to be further improved in luminescence efficiency withan increased initial luminance and a suppressed deterioration inperformance with time while preventing the adverse influence of oxygen.

The organic EL device according to the present invention may beapplicable to display apparatus, illumination apparatus, a light sourcefor a printer, a backlight of a liquid crystal display apparatus, etc.

When the EL device was used in combination with a simple-matrixelectrode structure or active elements (e.g., TFTs (thin filmtransistors)) to constitute a display apparatus, it becomes possible toprovide flat panel display with an energy saving effect, a highvisibility and lightweight properties.

When the EL device is used as a light source for a printer, it becomespossible to utilize the EL device as a laser light source for a laserbeam printer. In this case, for example, the EL device as the laserlight source is arranged in array to effect a desired light-exposure toa photosensitive drum, thus allowing image formation.

By the use of the EL device of the present invention, it is possible toremarkably reduce the size (or volume) of the above-mentioned apparatus.

Further, with respect to the illumination apparatus and the backlight, agood energy saving effect based on the use of a high-efficiencyluminescence device according to the present invention can be expected.

1-4. (Canceled)
 5. A luminescence device array comprising a substrateand a plurality of luminescence devices disposed on the substrate,wherein each luminescence device comprises a first electrode disposed onthe substrate, at least one organic luminescence function layer disposedon the first electrode, a second electrode disposed on said at least oneorganic luminescence function layer, and an oxygen absorbent, wherein aspace is defined between a first electrode of a first luminescencedevice and a first electrode of a second luminescence device arrangednext to the first luminescence device in one surface direction of thesubstrate, and wherein the oxygen absorbent is Mg and is disposed in thespace.
 6. A device array according to claim 5, wherein a voltage isapplied between the first and second electrodes to cause phosphorescencefrom at least one layer constituting said at least one organicluminescence function layer.
 7. (Cancelled).
 8. A device array accordingto claim 5, further comprising a sealing housing disposed on thesubstrate in order to cover the luminescence devices, and a hygroscopicagent which is sealed in a space between the luminescence devices andthe sealing housing.
 9. A device array according to claim 8, wherein thehygroscopic agent is CaO powder.